can prenup agreements be challenged?

Yes, some lawyers make very good money doing just that. 

They can be but you would need some good compelling reasons why the prenup was not a bona fide agreement between the two of you.

they sure can. all you need is leverage. word is, Melania redid her prenup. her leverage was she wouldn't come to D.C. She told the press it was until Baron finished school. she is one smart cookie! I like her.

Sure it can be challenged.  The real question you are asking is if it can be successfully challenged.

For that it would depend on the circumstances.  However,  unless there was a case of some specific fraud the chances would be slim.  If you are in this situation it would be best to consult an attorney.